Here’s why: Media stocks are up 24.4% over the last six months, outperforming the benchmark Standard & Poor’s 500, which rose 15.1% over the period. So they already reflect a lot of optimism. But investors may be disappointed by upcoming news about ad sales, ratings, and ticket sales, Nomura Securities analyst Michael Nathanson says in a report out this morning. His warning comes as media executives prepare to release their quarterly earnings and talk to Wall Street about the state of the business. They conduct hour-long infomercials designed to persuade the world that everything is fine — or, if it manifestly isn’t, that it’s someone else’s fault.

But Nathanson says the companies’ go-go projections about over-the-top ad sales miss how much Japanese auto makers are cutting production as they grapple with parts shortages following the country’s earthquake and tsunami. For example, Toyota will crank out 35,000 fewer cars than planned in North America in March and April. That’s a big deal: Auto companies typically spend about $1,200 on ads for each car they sell. Meanwhile, overall broadcast and cable network ratings stank in the first quarter. The Big Four networks’ live ratings were down 15.9% vs. the same period last year. As for movies, if you don’t know about this year’s miserable ticket sales, then you aren’t paying attention — although Nathanson says that box office would have been up 11% if last year didn’t include Fox’s Avatar and Disney’s Alice In Wonderland. Nathanson says that DVD sales everywhere outside of Walmart (which doesn’t release sales data) were down 16% compared to the first quarter of 2010.

The bottom line? Nathanson lowered his earnings estimates for Time Warner and Scripps Networks but raised them for Viacom and Discovery.